Electrical musical instrument



Feb. 25, 1941. L HAMMOND ETAL ELECTRICAL MUSICAL INSTRUMENT 3Shuts-Sheet 1 Filed Se t. 5, 1939- [/2 were fo /*5 La uf'ens' Hammonda'Jok/%1Zanerz" M 1% w. uqli 2 2 c w o I 7 M v LM 2 M 0W 3 Feb. 25,1941. L HAMMOND ETAL 2,233,258.

ELECTRICAL MUSICALYINSTBUMBNT Filed Sept. 5, 19:59 a sheets-sheet a QM E17: 65'

j/OO 7 [rm ere zors' a arena/Yam? on d e. Jo/m Mf/anerz" Patented Feb.25, 1941 UNITED STATES ELECTRICAL MUSICAL INSTRUMENT Laurens Hammond,Chicago, and John M. Hanert,

Wilmette, 111.; said Hanert assignor to Hammond Instrument Company,Chicago, 111., a corporationot Delaware Application September 5, m.Serial No. 203,444

23 Claims.

Our invention relates generally to electrical musical instruments andmore particularly to musical instruments of the melody type, andconstitutes an improvement on the instrument disclosed in theapplication of John M. Hanert,

Serial No. 274,325, flied May 18, 1939.

It is an object of our invention to provide an improved melody typeinstrument having a keyboard of a range oi several octaves by the use 01which any melody may be played.

A further object is to provide an improved electrical musical instrumentof the melody type employing a plurality of generators, and utilizing animproved keying system whereby undesirable j transients are suppressedboth when playing legato and non-legato.

A further object is to provide a keying system whereby the touch of theplayer exercises a control over the envelope of the tone.

A further object is to provide an improved electrical musical instrumentof the melody type having a plurality of frequency generators, akeyboard 0! a range greater than that of any one of the generators, andkey circuits controlled by the keys for selectively keying the signalfrom one or more of the oscillation generators.

A further object is to'- provide an improved keying system for anelectrical musical instrument in which relays are employed and in whichthe current flow through the winding of the relay has' an effect uponthe amplification of the output of the instrument independent of thecompletion of circuits by the relay.

A further object is to provide an electrical 35 musical instrumenthaving a variable gain electron discharge tube amplifier in its output,in

which the key completed circuits energize relays for causing thetransmission to the output oi the desired signal, and in which theenergization 40 of the relay, independent. of the switch contactsoperated thereby, controls the gain of said electron discharge device.

A further object is to provide an electrical musical instrumentemploying an oscillator, controlling the irequencies produced by theinstrument, with an improved vibrato producing means. v

A further object is'to provide an oscillator having improved means forperiodically shifting its frequency between selected predeterminedlimits, in which the mean frequency remains constant, irrespective ofwhich limits are selected.

A further object is to provide an improved relay keying system inconnection with a keyboard,

all the keys of which are free to be depressed at all times, without,requiring the use of mehaving no associated mechanically operatedswitches common to a plurality of said keys, to retard the maximum speedof action on said keys.

A further object is to provide an improved relay keying system for usewith a keyboard in which no serious restrictions are imposed on theplayer concerning the number of keys which may be simultaneouslydepressed in playing a melody such as, for instance, two keys beingmomentarily simultaneously depressed in playing from one to the other orsaid keys.

A further object is to provide an improved keying system for anelectrical musical instrument in which the time delay between theinitial current flow through the relay energizing coil and thesubsequent operation of contacts associated with the armature of saidrelay is utilized in a manner to eliminate undesirable transients fromthe output of the instrument.

A further object is to provide an improved keying system for electricalmusical instruments in which no substantial time delay occurs betweenbreaking a contact in series with a relay winding, and a change involtage occurring in a circuit associated with the relay winding.

A further object is to provide an improved keying system for electricalmusical instruments in which simplification and reduction of cost isefiected by providing a relay in which one function associated with theenergization of the relay is accomplished by taking advantage of achange in'voltage occurring in a circuit associated with the winding ofthe relay.

A further object is to provide an improved relay keying system for anelectrical musical instrument in which there is no period of silenceoccurring when two keys are simultaneously depressed in adjacentselected groups of keys.

Other objects will appear from thefollowing description, reference beinghad to the accompanying drawings in which;

Figure 1 is a perspective view of the instrument shown attached to apiano;

Figure 2 is a wiring diagram of the power supply system for theinstrument; and

Figures 3 and 3a together constitute a wiring diagram of the instrument.

General Description As shown in Figure 1, the keyboard for theinstrument comprises three octaves of the customary piano keyboard, thekeys being designated Ci to B8 inclusive. if The actual pitches of thetones produced upon depression of these keys may be varied as desired byregister changing switches such that the notes produced upon depressionof the keys may be selectively in the bass, tenor, or treble registers.As a result, the keyboard may, by the use of four such registercontrolling switches, be made to sound any note within a six octaverange,

In general, the instrument comprises a single master oscillator l0 whichis connected to control a series of six relaxation oscillators II to I!inclusive, which are connected in cascade and have their frequencies ofoscillation stabilized by a signal derived from the master oscillatorll. Instead of the relaxation oscillators disclosed herein, any othersuitable type of oscillator whose frequency may be readily stabilizedsuch as gas tube oscillators, multivibrators and the like. might beutilized. The frequency of oscillation of the master oscillator It maybe periodically shifted at a vibrato rate by means of a vibratomechanism 20, and this frequency shift of the master oscillator is ofcourse proportionally present in the frequencies of oscillation of thecontrolled oscillators i! to H inclusive. I

Each of the keys Ci to B3 controls the performance of several functions:

(a) It selects a tuning circuit for the master oscillator l0, and at thesame time selects the appropriate grid bias potential for the controlledoscillators ii to IT inclusive, so that these controlled oscillators areconditioned readily to be stabilized by the frequency of the masteroscillator;

.(b) Thereafter, it closes a,contact which energizes a relay, theenergization of which starts to turn the amplifier on with apredetermined rate of attack; a

(b) The energization of the relay causes completion of circuits whichrender the preselected tuning circuits effective, keys the signal fromthe appropriate oscillator to the amplifier, and causes a circuit to bebroken which renders ineflective the depression of another key in anadjoining octave group, which may -be depressed at the same time.

The instrument also includes a selectively operable switching mechanism24 for determining the register in which the instrument is played, andfor providing musical effects in which one or more octaves of the notemay be simultaneously sounded. The output of the instrument is suppliedto an amplifier 28, volume control 21, and

electroacoustio translating means. Power for operation of the variouscircuits is supplied by a power pack shown in Fig. 2.

Frequency Generating System a condenser C4.

pending upon the number of these condenser r ps which are connected inseries, tune the oscillator II to one of the semi-tone notes of anoctave. The frequencies thus generated constitute the semi-tones of thehighest octave of frequencies available in the instrument which are 2093to 3951 c.p.s.

Bias to tube ll is supplied by means of grid leak RI which is inparallel with a grid condenser Cgl. B creen bias to tube 3| is suppliedthrough voltage dropping resistor R2 and filter condenser Csl. Plateload on tube II comprises resistor R3, in series with winding II ofoscillation transformer 32, and resistance R4 which is connected tofilter condenser Cpi and filter resistance R5 to a terminal 300V of thepower supply, at approximately 300 volts potential. The suppressor gridof tube 30 is connected to the cathode.

The output of the oscillator Iii is coupled to the grid circuit of thecontrolled oscillator i2 through The oscillator i2 comprises a triode 38of the 6.156 type. the grid circuit of which includes a protectiveresistor R6, a secondary winding I of an oscillation transformer 42, andgrid resistor R1, which is connected to ground. The cathode of the tube3| is connected to a terminal HIV in the power supply system which is ata potential of 10 volts positive with respect to ground, there thusbeing 10 volts negative bias on the grid of tube ll.

Current is supplied to the plate of tube 38 through a conductor 44,which is connected to terminal 280V of the power supply system, througha timing resistor R8. The rate of relaxation of the oscillator I2 iscontrolled by the relative values of the timing resistor R8 and thecapacitance of a timing condenser C! which is in series with condenserC6 through the primary winding 4' of the transformer 42.

An output signal is derived from the oscillator l2 through a conductor Iwhich is connected to a point intermediate the condenser CO and thesecondary winding N. The output of the oscillator l2 appearing acrossthe condenser Cl will be generally of saw-tooth shape, thus comprising afundamental frequency with a long series of harmonic over-tones ofprogressively decreasing amplitude. The value of condenser CC is largerelative to that of Cl, and thus does not have an appreciable eflectupon the rate of relaxation of the oscillator i2- The constants of theoscillator circuits i2- and the potentials supplied thereto, are suchthat the relaxations occur at the same frequency as the masteroscillator II. In other words, the signal derived from the masteroscillator I0 and impressed upon the grid of the tube 38 through thecoupling condenser Cl serves to trip the relaxation oscillator for eachpositive impulse received.

The transformer 42 has a tertiary winding 5|! which is shunted by aresistance R9, of a value in the order of approximately ohms. Thesecondary ll may have approximately one half, while the tertiary windingSI may have approximately one eighth as many turns as the primary 46.The resistor R9 shunted across the tertiary winding is reflected intothe plate circuit of the tube 3! as a load impedance.

The oscillator I 3 may be substantially identical with the oscillator i2except that it will be noted that it is coupled to the oscillator I!through an adjustable tap 52 on the resistance R9. The adjustment of thetap I! will be such adjustment i made when the instrument is assembledand willnot ordinarily 'need to be readjusted. The biasing potential forthe oscillator I3 is supplied from a conductor I4 through a filterresistor RIO. A filter condenser C8 has one terminal connected to afixed potential terminal IV of the power supply, and its other terminalconnected to the tertiary winding I8 of transformer 42. The potential onthe conductor 84 is varied depending upon the key depressed, as willappear more fully hereinafter, so that the oscillator i8 will relax atone half the rate of oscillators i8 and I2, regardless of the frequencyto which the master oscillator I is tuned.

The relaxation oscillator i4 is similar to oscillator l8, except fordifferences in the values of the timing resistors R8 and condensers C5,and in the number of turns in the windings of its transformer 42. Therelaxation oscillator i4 also diiler from the oscillator I8 by theinclusion of a small inductance Li in series with the primary winding 46of the transformer 42 associated with the oscillator i4. This smallinductance serves to remove musically undesirable high frequencies inthe generally saw-tooth wave shape occurring across its condenser C8.

The oscillators i5, i8 and I! are similar to the oscillator l4, beingsimilarly connected in cascade, and having circuit elements of suchvalues that they relax at frequenciesin octave relationship. It will'benoted, however, that transformer 42 associated with the relaxationoscillator I! does not have a tertiary winding but instead, has a highresistance RH shunted across its primary winding to provide the plateload corresponding to that provided by the resistance R8 for therelaxation oscillator II.

The bias potential'upon the conductor 54, in order to make certain thatthe relaxation oscillators i8 to II inclusive safely follow changes infrequency of the relaxation oscillator i2, must be varied as thefrequency of the master oscillator in and of the oscillator I2 isvaried. The conductor 54 is connected to a point intermediate resistorsRiZ and Rl3 which together with a resistance R form .a voltage dividerconnected between the terminal 380V of the power supply and ground.Associated with these voltage divider resistors is a further voltagedivider resistance system consisting of resistors RCii to RB inclusive,connected in series between Ri2 and ground. The voltage appearingintermediate resistances RH and RI! will be determined by the particularkey which is depressed, as will be described hereinafter.

It is to be noted that tuning is accomplished by connecting selectedcondensers CC to CB to ground, and that such connection likewisedetermines which of the resistors R08 to RB are effective in the voltagedividing mesh provided by these resistors as well as the resistors Rl2,RI: and R. All of the resistors in this voltage dividing mesh are sochosen that resistors R03 to R3 inclusive do not constitute appreciableload (to alternating current of the master oscillator frequency) upontuning condensers CCt to CB, and Cct to Cb. In this way, the frequency.of oscillator ii) is determined substantially by the reactive elementsof the tuning system and not appreciably by the resistances RC5 to RB,RH, R13, or R, and thus good frequency stability for oscillator i8 isassured, be-' causeof the high Q (sharpness of resonance) of this tuningcircuit.

It will be noted that inductances L2 and L8 are inserted between thetuning condensers CD -and CD3, and CF and CH respectively, and thatthese inductances are wound together so as to form a mutual inductance Mbetween them. These inductances are very small compared to theinductance of oscillation transformer 32, and thus have inappreciableeffects upon the tuning of oscillator it. Their function is to suppressthe spark incident to tuning the oscillator i0.

It will be noted that when the number of effective condensers CC to GBin the tuning circuit is changed, the circuits by which such changes aremade are subjected to the residual D. Q. voltage on such condensers asare shunted out of the circuit by virtue of the bias voltage dividerresistors m to BB connected in shunt with them. The inductance L2 and L8either singly, or both mutually. depending upon which group ofcondensers is shunted, thus serve as a means to limit the discharge ofthe condensers to a rate such as will not cause excessive sparking atthe tuning contacts.

Vibrato mechanism The frequency of the oscillator III,' and hence of allof :the controlled oscillators I! to I! inclusive may, whenever desired,be periodically shifted throughout a small range, in .the order of 3%,at'a vibrato periodicity of approximately 7 c. p. s. to provide thevibrato in the tones ultimately produced.

The vibrato mechanism comprises a vibratory magnetic metal reed 58secured to a suitable rigid grounded support 50 and having a'weight 82adjustably secured thereto. so as to permit variations in the vibratoperiodicity. The reed 58 is maintained in vibration by an electro-magnet64, one end of the winding of which i connected to the terminal llillVof the power supply system. The other end of the winding of theelectromagnet 84 'is connected to a contact 66 which makes contact withthe reed during the upper portion of its vibratory oscillation, andsince the reed is grounded at the support 60, current flows through thecoil of the electro-magnet 64 and attracts the reed 58, whereupon itscontact with the contact 66 is broken.

A second contact 68 is provided for engagement with the reed 58 and isthus periodically grounded through the reed at the rate of vibration ofthe reed. This contact 88 is connected to single-pole double-throwswitches 10 and 12 by which the contact 88 may be made to interrupt thecircuit through a coil 14 of relatively high inductance, or through acoil 18 of lesser inductance. The coils "I4 and 16 are connected inparallel with approximately 13% of the turns of the primary 34 oftransformer 32 at a tap l8.

When either of the switches I0 or I2 is in its oif position, theinductance I4 or I8 associated therewith is connected to a tap 88 on theprimary 84 so as to cause the oscillator ill-to oscillate at a frequencywhich is the mean of the frequencies at which it oscillates when theinductance 14 (or 16) is periodically connected between the tap l8 andground by the contact 68. The number of turns between ground and tap I8is 29.1% of the number of turns between ground to tap 80. The switches18 and 12 are manually operable, and may be used singly or together, toprovide three different degrees of vibrato. The taps I8 Key circuits Aspreviously stated. and as shown in Fig. i, the instrument is composed ofa plurality of octaves of keys CI to B3 preferably of the generalarrangement utilized in piano and organ keyboards, and is hereinillustrated. as having a gamut of three octaves, although the keyboardcould be reduced or extended to any desired length. Since the instrumentis of the melody type in which more than two keys are seldom, if ever,simultaneously depressed intentionally, the keys CI to B3 may be verymuch shorter than the standard piano keys. By virtue of the size andpositioning of the keys they may readily be depressed simultaneouslywith the depression of the piano keys.

The keys CI to B3 are mounted in a suitable keyboard frame III which maybe conveniently attached to the front rail III of the keyboard of astandard piano, organ, or similar instrument. The frame ill carries aplurality of pivoted tablets it: which are utilized for the manualoperation of the various control switches,such as the vibrato switches10, I2, and the various other switches, described hereinafter, wherebythe tone quality is varied, the rate of attack of the tones controlled,and the instrument otherwise conditioned to produce tones of the desiredcharacteristics.

Above the tablets Ill is a channel-shaped guide I to receive blocks ll!bearing indicia identitying the tablets I83 as to the nature of thecontrols effected thereby.

It will be noted that the keys CI to B8 are spaced to register with thecorresponding piano keys, but are very much shorter than the piano keys.The two sets of keys are closely adjacent one another so that themusician may conveniently play simultaneoysly on both keyboards with onehand.

While the type 01 keys and keyboard disclosed herein is particularlywell adapted tor the control of a melody instrument, any other suitablekeyboard may be used. It should be noted, however, that when a pianotype of keyboard is used, the black keys should be made narrower thanusual so that a white key located between two adjacent black keys mayeasily be depressed without depressing either of the black keys.

Each of the keys CI to At! (except keys BI and B2), operates twoswitches in sequence. The switches which are closed first upondepression of these keys are illustrated in Fig. 3 as contacts C In toAlla respectively, while the contacts which are subsequently made bykeys CI to B3 are designated Clb .to Bib respectively. The contacts Oldto Mia are adapted to make contact with a bus bar 8|, while the contactsCla to Aila are adapted to make contact with bus bar I2, and similarly,the contacts Clo to Mia are adapted to make contact with a bus bar 83.All of the second made contacts of each of the-keys, i. e., .thecontacts Clb. to Blb are adapted, upon depression of the key, to contactwith a grounded bus bar It.

The contacts Cla to Aiila are connected respectively in parallel withthe contacts Cla to Mia and C3a to A330, and respectively to thecorresponding terminals of the condensers CC to CM. When no key isdepressed, the oscillator II is tuned to oscillate at the frequency ofthe note B (3951 c. p. s.) as determined by the capacitance of all ofthe tuning condensers taken in series. Therefore, when depressing a Bkey, it is not necessary to provide tuning contacts for makingconnection with the buses ll, I2 and 83. However,

it is necessary, for reasons which will appear for the second octave areconnected together by a conductor 80, and those for the third octave ofkeys by a conductor 81.

Associated with the first octave of the keyboard is a relay 8|, having abreak contact Ola and two make contacts llb and Ole. A similar relay 82having a break contact "a and two make contacts 02b and lie isassociated with the second octaves of the keys, while a relay 93 havinga similar break contact "a and two make contacts 83b and Me, is providedfor the third octave of keys. The windings of the relays ll, 92 and 93each have one end thereof connected to the conductors 85, 88 and .1respectively. The other end of the winding of relay II is connectedthrough the normally closed contact 92a and conductor III to a conductorBl which is connected to a positive potential terminal 300V of the powersupply through one section R98 of a voltage divider, the other sectionR88 of which is connected to ground, the section R99 being shunted by aconductor Cll.

Thus, upon depression 0! any of the keys in the first (low) octave, therelay 0| will be energized, providing the relay 92 has not beenenergized, and its contacts 92a are thus closed. Energlzation of therelay 9| results in the clo sure of contact Slb which connects the busii to ground, thus rendering effective the tuning of the oscillator IIIby the particular condensers taken in series, dependent upon theparticular key of the first octave which is depressed. At the same time,the closure of the contacts lb affects the grid bias of the controlledoscillators II to II inclusive, since the grid biasing resistors R08 toRB are connected in parallel with the tuning condensers. Due to the factthat these biasing resistors are in parallel with the tuning condensers,there will be a certain time lag whenever one of the playing keys isdepressed, before the biasing potential determined by the depressed keyis impressed upon the conductor 54, since a very small fraction of asecond is required for the discharge of the tuning condensers throughthe biasing resistors. This delay is, however, so short as not to be ofany significance because the resistors RCt to RB are of very low value,in the order of 5,000 to 15,000 ohms, and thus form a relatively lowresistance path for the discharge of these tuning condensers. On theother hand, the biasing resistors RC3 to RB cannot be made of too lowvalue compared to the impedance ofiered by the tuning condensers, asthese resistors would then lower the Q of the oscillator tuning circuit.

Energization of the relay 9! also results in the closure of the contactsIlc, one of which is connected to the output signal conductor I" whichis connected to the amplifier as will be more fully describedhereinafter. The other con tact of the contacts tic is connected to acon- Gil ductor IOI which is adapted to receive signals from thecontrolled oscillators I4 to I1 inclusive,

depending upon which oil a plurality oi register the signal outputconductors oi the controlled oscillators I2 to I1 inclusive throughamplitude controlling and non-robbing resistors RI04a, etc., associatedwith the switches I04a, etc., respectively.

'There is a conductor I02 tor the second or middle octave of keys, whichcorresponds to conductor IOI for the first or low octave of keys, and asimilar conductor I06 for the third or high octave of keys. When, forexample, the register control I04 is moved to the position in which itis shown in Fig. 3, the signal from the controlled oscillator I4 isimpressed upon conductor ml, the signal from the oscillator I3 isimpressed on the conductor I02, and the signal from the oscillator I2 isimpressed upon conductor I03.

Upon operating the register control I05, it will be noted that theconductors IOI, I02 and I03 will receive signals from controlledoscillators each one octave lower than when the register control I04 wasoperated. In a similar manner, operation of the register controls I06and I01 will cause signals to be impressed upon the conductors IOI,I02and I99 which are each two and three octaves lowerrespectively, thanare connected to these conductors, when the register control I04 isoperated. It will be noted that conductor I02 is connected to relaycontact 920 while conductor I03 is connected to relay contact 93c.

As previously pointed out, depression of a key in the lower octave, orfirst octave of keys, will not result in the energization of the relay9i if the relay 92 is energized. Similarly, the depression of a key inthe highest, or third octave of the keyboard, will not result in theenergize,-

tion of the relay 93 it the relay 92 is energized,

since it will be noted that one terminal of the winding of relay 93 isconnected to the conductor 61, while the other terminal thereof isconnected to the break contact 92a which, when relay 92 is energized, isopen.

Similarly, relay 92 cannot be energized unless both relays 9| and 93 arede-energized, because its energizing circuit leads from conductor 66through the winding 01 relay 92 to the normally closed contact 9I athrough conductor IIO, through the normally closed contact 93a and 9hence through conductor II2 to the conductor Amplifier and volumecontrols The conductor 96 leads to the; amplifier 26,

being connected to the cathodes of an intermediate push-pull stagethereof. The amplifier receives its signal through conductor I which isconnected to the grid 01' the first stage amplifying tube illustrated asa triode I20, and which may be of the 6J5G type, through a blockingcondenser CI2, the conductor I00 being connected to terminal IOV of thepower supply through a load resistor RI4. The grid of the tube I20 isconnected to terminal 2V oi the power supply through a grid resistorRIS.

The cathode of tube I20 is connected to terminal NW of the power supply,while its output or plate circuit is connected to the primary or atransformer I22, the other end of the primary being connected to theterminal 250V of the power pack. The secondary winding of the couplingtransformer I22 has its terminals connected respectively to the grids ofpush-pull amplifier tubes I24 and I26, which may be pentodes of the6K'7G type, having a remote cut off control grid characteristic. Thecathodes oi the tubes I24, I26 are connected'to a center tap of thesecondary oi transformer I22 through a condenser CI4, the cathodes alsobeing directly connected to the conductor 96.

The center tap on the secondaryof the transformer I22 has connectedthereto, through a resistor RI 6, a mesh for expression or volumecontrol. This mesh comprises a condenser CI6 having one terminalgrounded, and its other terminal I21 connected to the resistor RI6, anda resistor RI1 which is connected between the condenser CI and aconductor I28. The conductor I28 is connected to terminal'50V of thepower pack through a resistor RIB. The conductor I28 is connected toground through a. resistor Rlll and may, upon operation of theexpression'control lever I29, be connected to ground through one or moreof resistors R20 to R24, and when the swell pedal is closed (i. e. atlowest volume p0- sition), is connected to ground through a conductorI30.

It will be understood that upon operation of the swell pedal, .thecontacts on the resistors R20 to R24, and on the conductor I30, areclosed sequentially in the order named, and opened in the reverse order.In so doing, they change the potential uponthe conductor I26 from amaximum of approximately 50 volts to a minimum of ground, or zeropotential. The resistors RI8 to R24 inclusive thus form a variable.voltage divider circuit.

Resistor RI1 and condenser CI5 function to limit the rate of change ofpotential at terminal I21. Resistor RI 1 is in series with the swellpedal resistor mesh, and serves in conjunction with condenser CH, tocause the rate of potential change at the terminal I21 to beapproximately uniform as the swell pedal is moved from step to step. Thepotential at point I21 determines the grid bias applied to the controlgrids of push-pull tubes I24 and I26. These tubes are of the variable muor remote cutoil type, and advantage is taken of this feature inproviding a. simple swell or volume control system which will operateover a wide range such as 48 db., by varying the grid bias to tubes I24and I26.

By making gradual the rate at which this grid bias changes, the swell orvolume control-may be simplified into a relatively small number ofsteps, and still secure a wide range of volume without undesirablesudden changes in volume occurring when the swell or volume controllever I I! is moved from one position to another. Thus, if resistors RIOto R24 inclusive are so chosen as to provide successive 8 db. changes involume, a wide range of volume is provided with only six contacts. CI!is so chosen that volume changes are sufliciently gradual, but yet arenot too slow to prevent the volume from substantially following theswell pedal position to produce desired rapid dynamic changes in themusical tone being rendered. The control grid bias on tubes I24 and I28is determined not only by the swell pedal mesh previously described, butby the potential on the conductor 98 which, as previously described, isdetermined by the voltage divider mesh consisting of resistors R98 andR99. The

voltage of the conductor 98 is, howe er, not determined solely by theresistors R9! and R99, but is also changed whenever any of the relays 9I 82 and 93 is connected in shunt with R99 upon depression of any one ofthe keys.

When none of the keys is depressed, the voltage on the conductor 96 issuch as to bias the tubes I24 and I2. beyond cutoff irrespective of thesetting of the volume control. When, however, a key is depressed, thevoltage on the conductor 88 becomes more positive because of theshunting of the relay-winding impedance across RM, and thus theeilective bias on the grids of tubes I24 and I2! decreases, withresultant increase in the signal output of the tubes I24 and I2.

The rate at which the signal increases in the outputs of tubes I 24 andI26 is determined by the time constant 01' CM and RIG. When a playingkey is depressed, the control grids of tubes I24 and I24 remain atcutoil' potential because condenser CI4 is connected to these gridsthrough the transformer secondary and not through a resistor, whichmight limit the rate of change or potential. However, after the key hasbeen depressed, the transient voltage which was supplied throughcondenser CI4 will disappear and the bias upon the grids in tubes I24and I2. will.

CI4, and thus the rate at which the operating.

bias is applied to the control grids oi tubes I24 and I2. will beslower. Switch I32 may be manually operable by the player. whenreleasing the key, the rate at which tubes I24 and I26 cut oil isdetermined by condenser CIII which is in parallel with resistor R99. Itwill be noted that tubes I24 and I2. are connected in pushpull so thatchanges in the D. C. component 01' the plate current in these tubes doesnot haveany transient eilect upon their signal output.

The screen grids of the tubes I24 and I2! are connected to a terminalIUOV of the power supply system, while the suppressor grids thereof areconnected directly to the cathodes. Plate current is supplied to thetubes from a terminal IIIV through a conductor I34, which is connectedto the plates of the tubes I24 and I2 by load resistors R04 and R86respectively. The signal output of the push-pull amplifiers I24 and I2!is supplied to the grids of the power output tubes I40 and I42 connectedin push-pull arrangement through blocking condensers CI! and CIB.

The cathodes of the tubes I40 and I42, which may be pentodes oi the 6M6type, are sellbiased above ground potential through a common resistorR28, while the grids of the tubes I4! and I42 are connected to groundthrough grid resistors R30 and R12 respectively. The values of the gridresistors R" and R32 with respect to the values or condensers CH and Cl!respectively should be such that the sudden changes occurring in thepotentials on the plates of the tubes I24 and I26 will not result inaltering the potential on the grids of tubes I40 and I42 for anappreciable length of time.

The suppressor grids of the tubes I44 and I42 are internally connectedto their cathodes, while their screen grids are connected to theterminal 300V of the power supply system. The plates of the tubes I40and I42 are connected to the terminal 300V through a connection to themid-point on the primary 0! push-pull output transformer I44. Thesecondary oi the transiormer I44 is connected to the voice coils of aspeaker I48. The field coil 01' the speaker I46 may constitute aninductance in the power supply as will appear hereinafter.

Having described in general the construction 01' the amplifier, it willnow be more clearly understandable why the relays SI, .2 and 23 are soconnected that energizing one relay makes it impossible to energizeadjacent relays by depressing keys in adjacent octave groups of keys.

The reason for providing these mutual interlocking circuits is toeilectivelysuppress undesirable transients which might occur when theplayer, through a legato style of playing, causes two keys in adjacentoctave groups to be simultaneously depressed. 11' these interlockingcircuits were not provided, undesired signals or undesired changes intuning would occur. It will be noted that each time a player playsacross from one octave group of keys into an adjacent octave group ofkeys, the amplifier will momentarily be cut oil when the first mentionedkey is released, and then again rendered eiiective when the relayassociated with the second mentioned key is energized. Thus, the oldsignal is turned off with a controlled decay rate and the new signal isturned on with a controlled attack rate, as determined by the condenserCll, resistors R99 and RI. and condenser CI4. It will be noted that eachof the relays II, 4! and N is interlocked with its adjacent relay; butthat the relays 9| and I! are not interlocked and it is thereforepossible, it the player should depress keys in both the lowest and thehighest octave, to energize both relays II and I3. Such energi.

zation of these relays will cause the potentials on the cathod of tubesI24 and I2. to become very positive, causing tubes I24 and I2! todistort the signal. It is not contemplated, however, that the player inthe normal use or the instrument shall play legato from a note in thelowest octave to a note in the highest octave.

As previously described, upon depression of a key-for example, CI-itsswitch CIa will be closed before its switch Clb is closed, and uponrelease oi the key, the switch CI b will open before the switch Claopens. By having the biasing potential on tubes I24, I24 controlled bythe current flow through the windings of the relays SI, 82 and 93,thebias potential begins to change immediately upon opening of the switchClb, and will substantially reach cutoff potential at a non-transientproducing rate by the time that switch contact Chi is opened, and by thetime that the contacts 9| b and 3Ic open.

If the tubes I24, I25 were not biased substantially to cutoff by thetime switch CIa is opened, the oscillator I0 would begin changing itsfrequency to that of the note B. and thus an undesirable pitch changewould be heard during the interval between the opening of the switch CIaand the time that the biasing potential on tubes I24 and I 26 reachedcutoff.

Tone control system As shown in Figure 3, this tone control systemcomprises a plurality of resistors R34 to R39 inclusive, a pluralityof'inductances LIO to LI3 inclusive, a-plurality of condensers C20 toC23 inclusive, and controlling switches I50 to I55 inclusive. Theswitches I50 to I55 inclusive operate individually and selectively topermit the signal impressed on conductor I00 to develop across thecapacity-inductance resistance mesh associated therewith.

For example, when switch I50 is open, the signal on conductor I00 willhave no frequency characteristic impressed upon it because element R34is a pure resistance. When switch I5I is opened, the signal on conductorI00 will be altered in a manner to emphasize the low frequenciesrelative to the high frequencies. When switch I52 is opened, the signalon conductor I00 will be altered in a manner to emphasize the highfrequencies relative to the low frequencies.

When switch I53 is opened, the signal on conductor I00 will be modifiedsuch as to resonate a band of frequencies lying adjacent to the resonantfrequency of condenser C2I and inductance LII. The sharpness ofresonance is controlled by resistance R31 which is in parallel with C2Iand LI I. This resonant frequency may preferably be in the order of 3000c. p. s. Similarly, when switch I54 is opened, the signal upon conductorI00 will be resonated by condenser C22 and inductance LI2 and R38. Theresonant frequency here preferably is in the order of 1000 c. p. s. In alike manner, when switch I55 is opened, a resonant effect in the orderof 200 c. p. s. will be impressed upon the signal on conductor I00. Itis to be noted that these switches may be opened by the playerindividually or in any desired combination.

Operation of keying system I Upon depression of a playing key, forexample the key G2, the contact G2a is first closed, thus connecting allof the condensers CC and Co to CG and Cg, together with associatedbiasing resistors RCii to RG to the movable contact arm of relay switch92b.

However, such connection does not change the tuning of the oscillatorI0, since at this instant the relay contact 92b is open. Further depression of the key G results in closure of the switch (32b, whereupon acircuit from the grounded bus bar 84 through the switch 62b, conductor08, winding of relay l2, closed relay switch 9Ia, conductor IIO, closedrelay switch 03a, conductor II2, common conductor 38 and voltage dividerresistance R30 to power supply terminal 300V is completed. The flow ofcurrent through this circuit results in a lowering of the positivevoltage across the voltage divider resistance R99, and consequentreduction in the positive control grid bias on tubes I24 and I24, whichcauses plate current to start to flow through said amplifier tubes. 1

The energization of the relay 92 causes closure of the relay switch 32b,thus connecting the bus 32 to ground and thereby tuning the oscillatorI0 to the frequency determined by the previously mentioned condensers,which are thus effectively in the grid tuning circuit of the oscillatortube 30. This grounding of the conductor 32 also results in changing theground point of the series of biasing resistors RC1! to RB, and thuslowers the biasing potential on the conductor 54 to a' valuesufficiently more negative to condition the relaxation oscillators I2 toH to follow. safely the change in frequency of oscillator I0.

It will be understood that the grid bias on the relaxation oscillatortubes 38 is an important factor in determining the rate of relaxation,and that it is necessary to change the rate of relaxation of each of theoscillators I2 to I! through a range of substantially an octave.Changing the grid bias on the tubes 38 of the relaxation oscillator-saffords a simple method of simultaneously changing the frequency rangethrough which the relaxation oscillators I3 to I! may be effective. Theexact frequency at which the os clllators I2 to H will operate isdetermined by the amplitude and frequencies of the signals impressedupon the grids of their tubes. The condenser C4 may be of relatively lowvalue (e. g.

.003 mfd.) thereby to limit the amplitude of the signal impressed uponthe grid of the tube 30 forming part of oscillator I2, and to impart adesirable sharply peaked wave shape to this sig- 1311, to improve thestabilization of the oscillator Since the oscillator I2 operates at thesame frequency as the oscillator I0, the coupling afforded by thecondenser C4 is not very critical and in practice need not be madeadjustable. The amplitude of the signals impressed upon the grids of theremaining relaxation oscillators I3 to I! is adjustable during theconstruction of the instrument, by-adiustment of the potentiometers R3having the movable contacts 52. In practice, this adjustment is madesuch that equal increments of bias may be added and subtracted from thatimpressed upon the conductor 54 without causing loss of the frequencystabilization control, thus conditioning the relaxation oscillator forthe greatest safety as regards following the frequency changes of thepreceding oscillator, and maintaining exact frequency division by thefactor 2. For example, a small variable bias battery of 1 volts may beconnected in the conductor 54 adjacent the voltage divider RI 2 and RI3,and said battery alternately inserted with its potential such as to addor subtract from the potential provided by the voltage divider systempreviously described. If all of the relaxation oscillators I2 to I!continue to operate when the bias on the conductor 54 is thus changedthrough a range of 3 volts, the oscillators may be considered to beoperating safely. 1! any of the oscillators I2 to I! fail to divideproperly under the conditions where the voltage on the conductor 54 ischanged through this range of 3 volts, adjustment of the associatedpotentiometer R8 and R82 is made until such condition does obtain.

Thus, upon closure of the relay switch 821:, oscillator I8 will be tunedto the note G of the highest octave (3136 c.'p. s.), and the oscillatorI2 will likewise follow this frequency while the oscillators I3 to I Iinclusive will omillate at frequencies of 1568, 784, 392, 198 and 98 c.p. a. respectively Assuming that register control I88 has been operated.signals from the oscillators I8, I4 and I3 will be impressed upon theconductors I8I, I82 and I88 respectively. when the relay 82 is operatedand-the relay switch 820 closed, the signal from the oscillator I4 willbe supplied through theconductor I82 to the conductor I00, and hence beimpressed upon the grid of the amplifier tube I20 through the condenserCI2.

Energization of the relay 82 also opens relay switch 82a, thus openingthe current supply circuit tor the windings oi relays 8i and 88. As aresult, assuming that key G2 is held depressed, further depression of asingle key in either of the adjoining octave groups of keys CI toBI, or

C8 to B3 will have no effect. Depression of an additional key of thegroup C2 to B2 may, however, havea tuning eirect ii the second depressedkey is such as to lower the irequency of oscillator I8.

. It is of course contemplated that the player shall never press morethan two keys at one time. If, for example, while holding the key G2depressed the player should depress the keys C2 and 032, the condensersCC! and Get as well as the resistor RCt would be shunted through the bus82 and thus would alter the tuning of the oscillator I0.

Closure of relay switches 82b and 820 causes the signal of the properfrequency to 'be impressed on the grid of the amplifier tube I28.

Also, the amplification of tubes I24 and I28 is rising because of thechange in potential of conductor 88 at a desired rate of attack.Inasmuch as contacts 82b and 820 are made relatively soon after thevoltage is applied to relay winding 82,

.the amplification of tubes I24 and I28'will not have reached asubstantially high value at the time that closure of said relay contactsoccurs.

If, however, the relays operate relatively slowly, the potential on theconductor 88 and hence on the cathodes of the tubes I24, I28, may be ofsuch low value, when none of the relays is energized .(by properselectionof the values of the resistors R88 and R88), so that the tubesI24. I28 are normally biased considerably beyond cutofl. Thus, uponenergization of any of the relays 8|, 82, 88, a certain length of timewill be required for the potential on the cathodes of the tubes I24, I28to build up to the cutoff potential due to the time delay afforded bythe condenser CI4 and resistor RI8. This time delay may be correlatedwith the speed of operation of the relay armatures so that a conditioncannot arise that the amplifier tubes I24 and I28 have appreciable gainbefore the relay switches Me, 820 or 83c, as well as the tuning relayswitches 8Ib, 82b or 8217, have operated to complete their respectivecircuits. Thus, any transients incidental to the closure of saidcontacts ThlB,

gizing relay-82 and causing th potential on conductor 88 to rise tocutoiI value for tubes I24 and'l28. This rate of decay of operating biason tubes I24 and I28 is controlled by condenser Gill and resistance R88and is of such value as to provide a click-less decay rate. Thus, theampliiier'begins to decay immediately upon the opening of contact G2b.Following this operation and at a time when the amplifier issubstantially cut oil, the contacts 82a, 82b and 82c associated withrelay 82 are operated. In this way, all transients due to changing thetuning of the generator system and keying of the signals supplied bysaid generating system are eliminated, thus making the attack and decaymusically acceptable.

When the player plays a note such as the note G2 with the results Justdescribed, and while holding this key depresses another key, such as,for example, the key C8 in the higher octav of keys, the depression ofthe key C3 does not have any efiect until the key G2 is released.

The relay contacts are preferably so arranged that upon energization ofthe associated relay, the contacts 8Ia 82a and 83a are opened after themake contacts have been made, and may preferably take place as near aspossible to the end of the relay armature movement. Similarly, uponde-energization of the associated relay, its contact 8Ia, 82a or 83awill complete the circuit after the other relay switches have opened. Byreason of this sequence of operation of the relay switches, the shift ofthe tone from the note G2 to note 03 takes place in a minimum intervalof time-because relay is energized very shortly after relay 82 isde-energized upon release of the key G2. As a result, relay switches 82band 820 open substantially simultaneously with the closure of relayswitches 83b and 830. There mustbe' a slight interval between theseoperations in order to provide time for the change in bias on thepush-pull tubes I24, I28 through the conductor 88, and the amplifier issubstan tially cut of! during this interim oi time. The relays 8|, 82and 88 may be ordinary telephone type relays, but are, preferablyprovided with platinum or some other precious metal contacts.

For example, if the'player, while still holding the key G2 depressed,depresses another key in the same octave-for example key A2-the relay 82is not de-energized and upon release of the key G2, the oscillator IIIwill have its frequency shifted to the new frequency substantiallyinstantaneously upon release of the key G2. On the other hand, if, whileholding the key G2 depressed, he presses a lower note in the same otave-for example the note D2-the frequency of the oscillator I8 willimmediately be shifted to the new frequency of the note D even beforethe key G2 is released.

It is intended that the instrument be used to signals will appear onconductor I00 which will be associated with the signal contacts of eachof the "two relays BI and 03. Thus, the D note will sound as a low Dnote and another D note two octaves higher.

As a result of depressing these two keys, the voltage on conductor willbe more negative than ordinarily (because of the lowered impedance ofthe two windings of the into the signal, which distortion'products,however, willbe harmonics of the lowest of the two D signals. ,Under'these. conditions, the tones heard will be very loud, and produce a verybizarre efiect which will ordinarily not be desired, but may be utilizedupon rare occasions.

During the playing of the instrument, the swell or volum control maybe'operated at will successively to connect or disconnect the resistorsR to R24 in parallel, thus changing the, grid bias on the tubes I24 andI20, which results in a change in the amplification" factor of theseremote cutofi' tubes I24 and I20. Since the change in grid bias effectedby the volume control is made gradual by virtue of the condenser CI! andresistor RI1, the changes in the volume of the output arecorrespondingly gradual. Furthermore, any odd harmonic distortionintroduced by the curvature oi the grid characteristics of the remotecutofl tubes I24 and I20 tends to be substantially eliminated due to thepush-pull arrangement utilized.

Whenever the player desires to change the register in which the tonesounds, he may operate the desired register controls- I04 to I01 andthus shift the tonal register of the three octaves of keys provided. Ifdesired, two or more of these register controls I 04 to I01 may beoperated simultaneously, thus providing an effect similar to thatobtainedby the use of coupler switches in pipe organs, or that of morethan one instrument.playing simultaneously in octave or multi-octaverelationship. These register controls when two. or

more are used simultaneously, may be thought of as eilecting changes inthe tone quality. It will be understood that the register controls maybeof the type, such as pipe organ stop tablets, which remain-in eitherof their two positions to which they are moved.

The resistors RI04a to RI01a,'RI04b to R.I01b, and RI04c to RI 010 maybe of different values so 'as to provide means whereby the compoundingof' tones produced through simultaneously using a plurality of registercontrols may be-varied such that the octave coupling may be more or lessthan that which one would expect were it possible to play an octave onthe keyboard.

It will be understood of course that resistors RI04a, Rl04b and RI04cmust be of such value as to match in intensity the lowest note of oneoctave group to the highest note in the adioin-. ing octave group.However, resistors Rl04a and .RIIlSa, for example, may be chosen on thebasis Power supply system In the previousdescription of the instrument,various terminals have been referred to from which voltages and valuesrepresented by the reference characters of the terminals are supplied.Any suitable power supply capable of providing these voltages withreasonably good regulation may be used. In Figure 2, we have, however,shown a pmticular form of power supply system which is well adapted forthis purpose. It comprises generally aninput power transformer I60having a primary IOI which is supplied with 115 volt alternatingcurrent, being controlled by an on and 01 switch I02. The secondarywinding I62 of the transformer I80 has its terminals connected to theplates of a full wave rectifier tube 104 of a suitable type such as the5U4G tube, the filament of which is supplied with current by a tertiarywinding I. The heaters of the various tubes oi the instrument aresupplied by an additional secondary winding I68, the center tap of whichis connected to terminal I017.

The center tap of the winding I62 is impressed upon an input filtersystem comprising condensers C24 and C26, and an inductance LI4 whichmay be the field winding of the speaker I46. A protectiveresistance R40is connected between the filament of tube I64 and the condensers C24 andC28, which are connected to Terminals V and I50V' are connected tosuitable taps on' the voltage divider resistance R,

while the terminal 300V is directly connected to the conductor, I10. Theterminals 50V, I 50V and 300V are connected as previously described, to

supply power for the amplifier 28. It is to be expected upon closure ofone of the playing keys,

that the current drawn by the amplifier will change because" of thechanging load. This change in load will refiect itself as a change involtage between the conductors I10 and I12.

Relaxation oscillators I2 to 11 will relax at substantially the samefrequency provided that the ratio at their grid bias and plate biasremains constant. In order that the relaxation oscillators I2 to I1 willhave the ratio between their plate voltage and grid bias constant, aseparate voltage divider R42 is provided for the relaxation oscillators,being tapped at point I16, connected to terminal IOV, which serves as asource of cathode potential for the tubes 28. A condenser C28 isconnected between the tap I10 and the conductor I12 serving as a filterfor the from a terminal 2V, being provided by divider resistance R44,tapped at its mid-point, and filtered by resistor R48 and condenser C30.

The terminal 250V is connected to the conductor I10 through a filterresistor R48, and a filter condenser C32: connects the terminal 250V toground. The terminal 250V is utilized to supply the plate current andvoltage for tube I20.

.The terminal 280V is connected to the conductor I10 through a filteringresistor R50, and is connected to ground through a filtering condenserC34. The terminal 280V is the source for plate voltage for the tubes 38of the relaxation oscillator circuits. J

The terminal IO0V is connected to the conductor I10 through a filteringresistance R52 and connected to ground through afiltering condenser C36.the temiinal, IO0V being utilized to supply current-to the vibratomechanism for the vibration of the reed thereof.

General operation oscillator I0 will, provided no keys are depressed,

commence oscillation at the frequency of the note B in the highestoctave range, namely, at 3951 c. p. s., and the controlled oscillatorsI2 to I1 inclusive will of course oscillate at corre-,- sponding octavefrequencies below that of the oscillator I0.

As previously noted, the inductance 34 is variable. This inductance maybe adjusted preferably by changing the size of the air gap in the coreof the transformer. Such tuning will ordinarily be unnecessary since theoscillator ,I0 is of very stable construction and has excellentfrequency stability, but it may be necessary when it is desired to tunethe instrument to the pitch of some other instrument which cannot bereadily tu'ned.

The player may then select the register in which he wishes to play, byoperatingone or more of the register selectors I04 to I01. Thus, signalsfrom oscillators selected from the group I2 to H will ,be impressedselectively upon the conductors IOI, I02 and I03, which lead to therelay operated switches 8Ic, 92c, and 930 respectively. I

After making a suitable selection of the resonant filter circuits to beemployed, by opening one or more of the switches I50 to I55, theinstrument will be ready for use. When closing the on and ofi switchI62, the player will either independently or incidental to the operationof said switch, flick the vibrato reed 58 to cause it to commencevibrating. Thereafter the .reed

. will automatically continue its vibration. Before playing, theoperator, if he desires the vibrato tone, will operate either one orboth of the switches I0 and I2 to cause the desired vibrato frequencyshift in the frequency of oscillation of the oscillator I0. Likewise, ifa slow attack of the tone is preferred for the particular selection tobe rendered, the musician will close the switch I32.

Having thus set the various controls, the instrument is in condition forplaying, and is played as a melody instrument usually with an organlike,legato touch. 1

As previously mentioned, any suitable frequency generating system usingany one of various types of oscillators may be employed in theinstrument in place of the frequency generating system disclosed herein.The frequency generating system per se does not form a part of ourinvention claimed herein, except in combination with the remaining partsof the instrument.

Our invention has been described as relating to a melody instrument, bywhich we mean an instrument in which ordinarily but one note is soundedat one time so that it is not .generally capable of playing chords. Theinstrument disclosed herein is thus primarily a melody instrument asthus defined, although, as previously stated, chords consisting of notesin octave relationship may be played upon the instrument. Furthermore,by the use of several manuals and duplication of other parts of theinstrument, it will be readily apparent that an instrument capable ofplaying chords could readily be constructed. It is with these facts inmind that the instrument is generally described herein as a "melodyinstrument.

While in a number of instances various elements of the circuit have beenspecified as being of particular values, these values may, as is wellknown in the art, be varied considerably, especially if compensatorychanges are made in the arrangement or values of other parts of theinstrument.

It will be apparent to those skilled in the art that numerousvariations, modifications claims. to. include within the scope of ourinvention all such modified constructions whereby substantially theresults of our invention may be obtained by substantially the same orequivalent means.

We claim:

1. In an electrical musical instrument, the combination of a pluralityof groups of keys, 9. relay associated with each group of keys, circuitsfor energizing each of said relays upon depression of any of the keysof'its associated group, and means operated upon energization of any oneof said relays to open the energizing circuits of relays associated withthe groups of keys located immediately adjacent the group of keysassociated with the energized relay.

2. In an electrical musical instrument, the combination of three groupsof keys, a relay associated with each group of keys, circuits forenergizing said relays respectively upon depression of any key in-thegroup associated with the relay to be energized, and means operated bythe energized relay'for preventing the energization of the relayassociated with each group of keys located adjacent the group of keysassociated with the energized relay.

3. In an electrical musical instrument, the

combination of three groups of keys, three relays associatedrespectively with said groups of keys, a relay energizing circuit foreach of said relays, switches operated by the keys of each group forcompleting the circuit for energization of the relay'associatedsuchgroup, normally closed switch contacts operated by said relaysrespectively, said normally closed switch contacts of one relay being inat least one of said circuits for energizing another of said relays.

4. In an electrical musical instrument, the combination of three octavesof keys, a relay associated with each octavecf keys, switch contactsassociated with each relay and opened upon energization thereof, and anenergizing circuit for each of said relays arranged to be completed upondepression of, any one of the keys of the octave associated therewith,each of said circuits including at least one of said switch contacts ofone of the other relays.

5. In an electrical musical instrument, the combination of a pluralityof keys divided into groups, a relay associated with each group of keys,circuits for energizing said relays upon depression of any key of thegroup associated with the particular relay to be energized, switchcontacts opened upon energization of said relays respectively, andcircuits connecting the switch contacts of the relay associated'with onegroup of keys inthe relay energizing circuits for relays associated withadjacentgroups of keys, whereby upon energization of a relay associatedwith one group of keys the depression of a key in an adjacent group willbe melody type, the combination of a plurality of octave groups of keysfor controlling the transmission to the output oi the instrument ofsignals irom sources supplying signals of musical. irequency, a relayfor each octave group of keys, an energizing circuit for each oi saidrelays arranged to be rendered efi'ective upon operation of any 0! thekeys inv its associated octave group of keys, and means operated by eachof said relays upon the energization thereof to prevent the energizetionof a relay associated with an adjacent octave group of keys.

'I. In an electrical musical instrument having a plurality ofinter-stabilized electrical impulse generators providing impulses atfrequencies bearing octave relationship with respect to one another, andhaving an electron discharge tube amplifier and electroacoustictranslating means supplied with the output of said amplifier, thecombination of a plurality of manually depressible keys, a pair ofswitches operable sequentially by each of said keys, a relay, anenergizing circuit for said relay including one of said key-operatedswitches, means responsive solely to current fiow through said relayenergizing circuit to cause a controlled rate of change in theamplification by said electron discharge device, and a switch operatedby said relay to connect one of said sources to the input circuit ofsaid electron discharge device.

8. In an electrical musical instrument of the melody type having aplurality of manually operable keys, tunable sources of electricalimpulses o! diilerent musical frequencies, and having an amplifier andloud speaker; the combinationof a switch operated by each of said keysand effective to condition said-sources to be tuned to frequenciescorrelated with the nominal frequency corresponding to the note of thekey depressed, a relay having an energizing circuit and a pair ofcontact switches which are closed upon energization of the relay, anenergizing circuit for said relay including a second switch operated bysaid key, means controlled by one of said relay contact switches toconduct a signal from one oi! said sources to said amplifier, a circuitcontrolled by the other of said contact switches for rendering effectivethe tuning of said sources, and means rendered efiective by current flowthrough the energizing circuit of said relay to change the gain of said9. In an electricalv musical instrugnent havin to connect one of saidgenerators to the input circuit of said electron discharge tubeamplifier;

10. In an electrical musical instrument, the combination of a pluralityof groups of keys, a relay associated with each group of keys. circuitscontrolled by said keys of each group for energizing the relayassociated therewith, and means operated by each relay upon theenergization thereof to open such or said circuits as energize therelays associated with adjacent groups of keys.

harmonicaily related frequencies, means to tune said generators tofrequencies or harmonically related notes 01' the musical scale, aplurality of groups of keys, a switch operated by each of said keysrespectively to condition said tuning means to tune said generators topredetermined frequencies, a relay having a winding and a pair ofcontact switches closed upon energization of said winding, a circuitincluding one of said contact switches for rendering effective saidtuning means, a circuit completed by said second contact switch tosupply electrical impulses from one of said generators to saidamplifier, means normally effective to bias an electron discharge tubeof said amplifier beyond cutoi'i', means to complete an energizingcircuit through the winding of said relay upon depression of any one ofthe keys of one of said groups, and means for changing the bias on saidtube at a controlled rate as aresult of current fiow through the windingof said relay.- I

12. In an electrical musical instrument having an output amplifierincluding amulti-electrode electron discharge device, a voltage dividercircuit normally supplying a potential to said device sufiicient to biassaid device substantially to cutoil', a'relay controlling thetransmission of a signal to said amplifier, and a key controlledenergizing circuit for said relay, said circuit being in'parallel with aportion of said voltage divider circuit, the impedance 01 saidenergizing circuit 1 being oi. such value with respect to the imped anceof said portion of said voltage divider circuit that the bias on saidelectron discharge device will be reduced to a value above cutoffwhenever said energizing circuit is completed.

.13. In an electrical musical instrument of the melody type having amuti-octave range key-- board, a plurality oi electron discharge devicegenerators of musical signals bearing octave frequency relationship toone another, means controlled by keys of corresponding note designationof difi'erent octaves simultaneously to tune all oi said generators tofrequencies corresponding to such note designation, an amplifyingelectron discharge device, means normally biasing said devicesubstantially to cutofl, a relay having an energizing circuit closed bydepression of any of the keys '0! an octave of said keyboard, meansaffected by current flow through said circuit for decreasing the bias onsaid device at a predetermined rate, and a circuit completed uponenergization of said relay to impress a signal from one of saidgenerators upon said amplifying device.

14. In an electrical musical instrument having a plurality of octavegroups of keys, a plurality of generators, an amplifier, a plurality ofcircuits controlled by said keys for connecting said generators to saidamplifier, and means operated upon completion of one of said circuitsunder the control of a key of one of said groups to prevent thecompletion of another of said circults under the control of a key in anadjacent group.

15.,In an electrical musical instrument, an amplifier including anelectron discharge device having a cathode, plate and control grid, avoltage divider determining the operating potential of said cathode. arelay, a circuit controlled by said relay for impressing a musicalsignal upon the grid and cathode of said device, and a keycontrolledcircuit for energizing said relay, said key-controlled circuit being inparallel with a portion of said voltage divider, whereby the potentialof said cathode is changed upon completion of said key-controlledcircuit.

16. In an electrical musical instrument having means for generating andcontrolling a musical signal and having an amplifier includingamulti-electrode electron discharge device for receiving a signal fromsaid generating and controlling means and transmitting the signal toelectroacoustic translating means; means to control the amplitude of thesignal output of said device comprising, a terminal, a source of directcurrent potential adjustable by relatively few and large increments,said adjustable source being connected to said terminal, means readilyoperable by the player of the instrument for adjusting the potential ofsaid source, a reactive element connected between said terminal and apoint of fixed potential, said element being of sufiicient reactance tocause the rate of change of potential on said terminal to changesmoothly despite abrupt changes in the potential of said source, andmeans connecting said terminal to one of the electrodes of said deviceto determine the bias thereof.

1'7. In an electrical musical instrument having means for generating andcon-trolling a musical signal, an amplifier including a variable-muvacuum tube for receiving said signal and transmitting it toelectroacoustic translating means; means to control the amplitude of thesignal output of said tube comprising, a terminal connected to thecontrol grid of said tube, a voltage divider having a connection to saidterminal for determining the potential thereof, said voltage dividerbeing adjustable only by relatively large steps, means [readily operableby the player of the instrument for controlling the adjustment of saidvoltage divider, and a reactive element connected between said terminaland a point of fixed potential, said reactive element being ofsufilcient value to cause the potential on said terminal to changegradually from one potential value to another upon abrupt adjustment ofsaid voltage divider from one position of adjustment to another.

18. In an electrical musical instrument, the combination of anoscillator having an adjustable tuning circuit including a plurality ofcondensers connected in series, voltage dropping resisters connected inparallel with said condensersrespectively, key operated switchcontrolled cir cuits for selectively shunting groups of said condensers,and a plurality of inductances of relatively low impedance connected inseries with said condensers between groups thereof, whereby thedischarge of condensers shunted by said key controlled circuits will beimpeded to reduce arcing upon opening and closing said key operatedswitch controlled circuits. v

19. The combination set forth in claim 18 in which two of saidinductances are provided and lay energized upon closure of said secondkey operated switch, said relay being eifective to close the otherswitch of said pair of switches.

21. In an electrical musical instrument having means for generating andcontrolling a musical signal, and an amplifier including an electrondischarge device for receiving said signal and transmitting it to anelectroacoustic translating means; means to control the amplitude of thesignal output of said device comprising, an element readily operable bythe player of the instrument, a plurality of fixed impedances formingpart of a voltage controlling circuit and arranged to be successivelyconnected in the circuit by said element, a terminal the potential ofwhich is determined by the number of said impedances which are connectedin said circuit, a reactive element connected between said terminal anda point of fixed potential to limit the rate of change of potential onsaid terminal, and means connecting said terminal to one of theelectrodes of said electron discharge device to determine the biasthereof.

22. In an electrical musical instrument having an oscillator thefrequency of oscillation of which is determined in part 'by the amountof inductance in a tuning circuit thereof, the combination of a coilhaving end terminals and intermediate taps, said coil forming the tuninginductance of the oscillator, a pair of inductance elements normallyconnected in parallel across said intermediate taps, a periodicallyoperating interrupting switch having one contact thereof connected toone of said terminals, and manually operated switches separatelyoperable to disconnect a terminal of each of said inductance elementsrespectively from an intermediate tap of said coil and connect it to theother contact of said interrupting switch.

23. In an electrical musical instrument having an oscillator thefrequency of oscillation of which is determined in part by the amount ofinductance in a tuning circuit thereof, the combination of a coil havingend terminals and intermediate taps, said coll forming the tuninginductance of the oscillator, a pair of inductance elements normallyconnected in parallel across said intermediate taps, and meansselectively rendered effective to connect either one or both of saidinductance elements periodically in parallel with the portion of saidcoil between one terminal thereof and the tap thereof nearest saidterminal.

LAURENS HAMMOND. JOHN M. HANERT.

